50 PLANT GROWTH 



depths. The "field capacity" is a term used to express the 

 amount of water held by a soil to a given depth. Only addi- 

 tional water will pass to the deeper soil, and will increase the 

 depth to which the field capacity has been reached. This will 

 be considered later under the heading of artificial watering. 



The mineral salts in solution in the capillary water will 

 move with the water, but they also move by diffusion. If a 

 root hair is absorbing a particular ion (the smallest unit of 

 an absorbed substance, as an ion of potassium) faster than 

 it is absorbing the water in which it is in solution, the concen- 

 tration of the ion will be decreased which will cause that kind 

 of ion to move by diffusion toward the root hair at a greater 

 speed than the solution. In other words, ions and water 

 molecules enter the root hairs as individuals by diffusion, and 

 they may move that way in the water films around the soil 

 particles instead of moving as a group of mixed molecules, 

 which make a solution such as a drop of a sugar solution. 



The water-supplying power of any soil refers to the 

 amount and the rate at which the plant can absorb water 

 from the soil. Since the water moves by capillarity in the 

 soil, it is clear that a plant can get from the soil the water 

 immediately around the root hairs and the water that will 

 move to the root hairs. A soil of finer texture, other condi- 

 tions being the same, will hold more water, in proportion to 

 the increase of particle surface in contact with a root hair, 

 than a coarser soil. It is evident for this reason that a fine 

 soil will normally supply more water to plants than a sandy 

 soil and that plants will wilt sooner in a sandy than in a fine 

 soil. 



The air of the soil is necessary to supply oxygen to roots 

 and to the organisms causing decay. Protoplasm must have 

 oxygen to grow or in fact to live. A loose porous soil with 

 a low water content will contain a large quantity of air. The 

 oxygen can diffuse into this air as it becomes depleted of 

 oxygen by the roots. However, a soil with too much water 



